This invention relates to a circuit and a method for controlling the contrast in a digital television receiver, in which the level of a composite video signal is converted such that a pedestal value of the composite video signal has a zero level.
In general, the contrast control in a digital television receiver refers to controlling the shading (or brightness) of picture on a screen. In a general analog television receiver, there is proposed a method commonly used for converting the amplitude of the brightness signal by means of adding or subtracting the gain of the image signal component provided from an image amplifier. As far as, however, a digital television receiver which is widely researched nowadays is concerned, it is necessarily demanded that the contrast control against a video signal converted into digital signal should be carried out proportionally with respect to the amplitude change of an original composite video signal, and further it should be the composite video signal.
The digitalized signal of the video composite signal is in general related to the bit number of an analog-digital (A/D) converter employed to a digital contrast circuit. As illustrated in FIG. 1, the signal output of the A/D converter is binarilly processed to have top value 101 and bottom value 104, dividing the interval between the top value 101 and bottom value 104 into given levels of n-bit data (in case of n-bit A/D converter). For instance, in a digital contrast circuit employing a 4-bit A/D converter, the top value 101 will be of binary 1111 and the bottom value 104 will be of binary 0000 in the straight code. Furthermore, the 2's complement values of each the above values are respectively 00001 for the top value 101 and 1000 for the bottom value 104.
In addition, the value of the intermediate level 102 can be described in a given digital value. For example, if the straight code of the n-bit intermediate value 102 is of 1000, then the 2's complement code corresponding thereto will be of 1000.
In this case, a signal portion 107 of brightness signal component of the composite video signal 105, relating to the contrast control, is placed between the pedestal level 103 of the composite video signal 105 and the peak value 106 of the composite video signal. In other words, the brightness signal component relating to the contrast control is a signal portion of the composite video signal excluding the synchronous signal component and the pedestal level 103. Namely, it is the signal between the pedestal level 103 and the top value 101.
If a method of multiplying the gain of the contrast signal for the entire composite video signal, without using the above mentioned method, is used for controlling the contrast signal, then the level of the synchronous signal and pedestal signal component are also multiplied with a same ratio as the contrast signal component, which causes the distortion of the composite video signal. FIG. 2B shows distortion of the composite video signal against the original composite video signal of FIG. 2A.
Of the distorted composite video signals illustrated in the drawings, distortion of the synchronous signal component may be avoided by suppressing the multiplication during the synchronous signal by means of a control gate. Nevertheless, the distortion of the DC component of the pedestal level 103 can not be avoided. Accordingly, it is in a great necessity to stabilize the pedestal signal level excluding the distortion of the original composite video signals, controlling independently only the signal component relating to the contrast signals.